Low power data communications systems capable of non-licensed wireless communications are already in practical use. Such systems do not need any license and are convenient to use. However, when such systems are being used, it is difficult to find out who are using them and how they are being used. Therefore, conventionally, such systems try to detect an interference, and if an interference is found, communications are performed through other channels or stopped until the interference disappears (see, for example, Japanese Laid-Open Applications No. 2001-45538, No. H5-300047 and No. H5-206942). Therefore, additional communications become practically impossible to be performed with such systems when the number of users thereof reaches a certain level.
This is inevitable in that, since any non-occupied channels can be used by anybody, users of the wireless systems for non-licensed communications, unlike ones for licensed communications, cannot secure specific communications channels for their exclusive use, causing inconveniences to the users thereof.
The conventional wireless systems for non-licensed communications has a drawback in that, even if a new user thereof has all the apparatuses for implementing such systems installed, there exists a possibility that non-occupied channels may not be available for the new user's use depending on the channel situation. Therefore, there have been demands for a method for increasing the number of effective channels within a limited frequency range.